Patent Application
20130008394
1. Field of the Invention
The present invention relates to a radiant superheater. More particularly, the present invention relates to a radiant superheater arranged to hang at the upper portion of a furnace of a boiler, especially, of a supercritical or ultra supercritical pulverized coal firing boiler.
2. Description of the Related Art
In practice, all power boilers of today have superheaters (SH), i.e., heat exchangers, in which the temperature of high pressure steam produced in an evaporator is raised above the saturation temperature. Superheated steam is then conducted to a high pressure steam turbine to produce steam power. Many boilers also comprise a reheater, in which the temperature of lower pressure steam released from the high pressure steam turbine is raised again, in order to produce more power by an intermediate pressure steam turbine. In the following, the term “superheater” may refer to either an actual superheater or a reheater. The term supercritical (SC) boiler refers to a boiler having a steam temperature of at least about 550° C., whereas, for ultra supercritical (USC) boilers, the steam temperature is at least about 600° C. The use of increased superheat and reheat temperatures improves the cycle, and thus, the plant efficiency.
Usually, a power boiler comprises a superheater system consisting of multiple in-series-connected superheater sections, which are located in different parts of the boiler. Superheaters are generally called convective superheaters (CSH), into which heat is mainly conducted by hot flue gas, or radiant superheaters (RSH), which dominantly absorb heat by radiation. Radiant superheaters are arranged at the top of the furnace of a boiler to be in direct visibility to the flames in the furnace. For SC and USC pulverized coal firing boilers, the duty of the RSH is substantially greater than that for a supercritical boiler. Thus, a series RSH arrangement is often used to obtain the required steam enthalpy. The metal tube temperature of an RSH depends on the local heat flux and on the temperature of the steam flowing in the tube. The metal temperature can be especially high at the bottom of a radiant SH, facing the flame zone.
Superheating of saturated steam is usually started in a CHS arranged in the flue gas channel downstream of the furnace. From the CHS, the steam usually goes to an RSH arranged at the upper portions of the furnace. The RSH may comprise pendant tube coils or hanging panels of tubes, or divisional tubewalls arranged parallel to the flue gas flow. Steam leaving the RSH usually goes to an attemperator, where water is sprayed onto the steam, to bring down the steam temperature to its desired value. From the attemperator, steam finally goes to a pendant superheater (PSH) arranged behind the nose of the furnace or in a horizontal pass immediately downstream of the furnace for further superheating the steam before it leaves to a high pressure (HP) turbine. Steam exiting the HP turbine may be conducted back to the furnace for being re-superheated to the desired temperature in a reheater (RH). Steam, after being reheated, flows to the intermediate pressure (IP) turbine for further expansion. The RH is usually arranged in the horizontal pass downstream of the PSH, but it may, as well as the PSH for final superheating, in some cases, also be arranged as a radiant superheater at the top portion of the furnace.
Due to high flame temperature in the furnace, the durability of radiant superheaters may suffer from overheating. German Patent No. 1012614 discloses an arrangement in which the tubes of a radiant superheater are protected from overheating by special shield tubes leading steam to a convective superheater. Great Britain Patent No. 855,114 discloses a boiler having superheater tubes, closest to the flame in the furnace, protected from radiation by reheater tubes surrounding the superheater tubes. It is also known from U.S. Pat. No. 3,101,698 to make a platen superheater behind a furnace nose, in which third and fourth passes are arranged partially in parallel flow, i.e., so that horizontal radiation is directed to tubes of a third pass, which are in flue gas flow upstream of the fourth pass, to prevent overheating of the hotter outlet tube sections.
The above-mentioned prior art solutions may adversely alter the heat duty among the superheating stages, and thus, lower the thermal efficiency of the boiler, or they address primarily convective dominant heat transfer. Therefore, there still exists a need for an improved radiant superheater.
An object of the present invention is to provide a radiant superheater by which the problems of the prior art mentioned above are minimized.
According to one aspect, the present invention provides a radiant superheater arranged to hang at the upper portion of the furnace of a boiler, wherein the radiant superheater is substantially planar and comprises a first vertical pass, a first connection pass, a second vertical pass, a third vertical pass, a second connection pass and a fourth vertical pass, each vertical pass comprising an upper end and a lower end. The vertical passes are connected in series, so that steam to be superheated enters at the upper end of the first vertical pass and flows through the first vertical pass and from the lower end of the first vertical pass via the first connection pass, to the lower end of the second vertical pass and through the second vertical pass, and from the upper end of the second vertical pass to the upper end of the third vertical pass and through the third vertical pass, and from the lower end of the third vertical pass via the second connection pass to the lower end of the fourth vertical pass and through the fourth vertical pass, to be discharged from the upper end of the fourth vertical pass. The first connection pass is arranged below the second connection pass, so as to shield the second connection pass from radiation from the lower portion of the furnace.
Generally, a flow of relatively low temperature steam comes from a primary, convective superheater (CSH), arranged in the flue gas channel to the first vertical pass of the radiant superheater (RSH). Due to the continuous radiant heating of the RSH, the average temperature of the steam is constantly increasing when the steam is flowing from the first vertical pass up to the fourth vertical pass. Therefore, the temperature of the steam in the first connection pass is lower than that of the steam in the second connection pass. Correspondingly, even if the level of radiation at the outer pass is at its maximum, the metal temperature of the outer pass can be maintained at an acceptable level, due to the relatively low temperature of the steam flowing in the first connection pass.
According to a preferred embodiment of the present invention, the first vertical pass and the second vertical pass are arranged to surround the fourth vertical pass and the third vertical pass in a horizontal direction, so as to, in the horizontal direction, shield the fourth vertical pass and the third vertical pass from the radiation. Thereby, the first and second vertical passes and the first connection pass, inside of which, flows a relatively low temperature steam, are arranged outside the third and fourth vertical pass and the second connection pass, inside of which, flows a higher temperature steam. Thus, the most intense radiation from the flames is in all relevant directions directed to the tubes of the first and second vertical passes and of the first connection pass, whereby the peak metal temperature of a radiative superheater is reduced. This is particularly important in an SC or a USC boiler, where steam of an especially high temperature is generated.
Usually, each of the first vertical pass, the first connection pass, the second vertical pass, the third vertical pass, the second connection pass and the fourth vertical pass comprises multiple parallel steam tubes. The multiple parallel steam tubes of the first vertical pass, the first connection pass and the second vertical pass preferably form continuous U-tubes, i.e., each of the multiple parallel steam tubes of the first vertical pass are in steam flow connection to one of the multiple parallel steam tubes of the first connection pass, and each of the multiple parallel steam tubes of the first connection pass are in steam flow connection to one of the multiple parallel steam tubes of the second vertical pass. Correspondingly, the multiple parallel steam tubes of the third vertical pass, the second connection pass and the fourth vertical pass preferably form continuous U-tubes, i.e., each of the multiple parallel steam tubes of the third vertical pass are in steam flow connection to one of the multiple parallel steam tubes of the second connection pass, and each of the multiple parallel steam tubes of the second connection pass are in steam flow connection to one of the multiple parallel steam tubes of the fourth vertical pass.
According to a first preferred embodiment of the present invention, the second vertical pass and the third vertical pass are connected in series in such a way that steam flows from the upper end of the second vertical pass via a third connection pass to the upper end of the third vertical pass, in which the third connection pass comprises multiple parallel steam tubes. The third connection pass may be arranged above the roof of the furnace, but preferably, it is arranged within the furnace, in order to absorb heat also in the third connection pass. Advantageously, each of the multiple parallel steam tubes of the second vertical pass are in steam flow connection to one of the multiple parallel steam tubes of a third connection pass, and each of the multiple parallel steam tubes of the third connection pass are in steam flow connection to one of the multiple parallel steam tubes of the third vertical pass. Thus, the multiple parallel steam tubes of the second vertical pass, the third connection pass and the third vertical pass form multiple parallel inverted U-tubes. Thus, the above-mentioned way of connecting the tubes provides the advantage that tubes that are at the outer edge of the second vertical pass, i.e., those that receive the most radiation heat, continue as tubes at the inner edge of the third vertical pass, i.e., they receive the least amount of radiation heat. In this way, the heat pick-up of the tubes is balanced.
According to a second preferred embodiment of the present invention, the upper end of each of the multiple parallel steam tubes of the second vertical pass is connected to a first header arranged above the roof of the furnace, the upper end of each of the multiple parallel steam tubes of the third vertical pass is connected to a second header arranged above the roof, and the first header is in steam flow connection with the second header by a connecting pipe arranged above the roof. Preferably, the connecting pipe comprises a water attemperator, whereby it is possible to adjust the temperature of the steam to a desired level before it enters the third vertical pass.
According to a further preferred embodiment of the present invention, the first vertical pass is arranged adjacent to the front end side of the boiler, and the second vertical pass is arranged adjacent to the back end side of the boiler. Taking into account that the radiant superheater also partially acts as a convection superheater, the use of the above-mentioned arrangement provides the advantage that flue gas mainly flowing from the front end side towards an exit at the back end side hits first the first vertical pass, where the steam temperature is the lowest. Thereby, the risk of overheating the second vertical pass is minimized.
According to an advantageous design of the above-mentioned embodiment of the present invention, the fourth vertical pass is arranged adjacent to the first vertical pass, and the third vertical pass is arranged adjacent to the second vertical pass. In this way, due to the role of the radiant superheater as a convection superheater, the final temperature of the steam is raised. It is also possible, and in some applications, advantageous, to switch the third and the fourth vertical passes, i.e., to arrange the third vertical pass adjacent to the first vertical pass and the fourth vertical pass adjacent to the second vertical pass.
The radiant superheater is advantageously an intermediate superheater arranged in steam flow direction upstream of a high pressure steam turbine. This means that hot steam from the superheater is led to a finishing superheater before it is conducted to the high pressure steam turbine. Alternatively, the superheater may be a finishing superheater, from which the steam is directly conducted to a high pressure steam turbine. It is also possible that the radiant superheater is an intermediate or finishing reheater, i.e., a superheater arranged in the steam flow between a high pressure steam turbine and an intermediate pressure steam turbine.
The present invention, as described above, relates to a consideration of the metal temperature of a radiant superheater by having the highest local heat flux combined with a relatively low steam temperature inside the tube. For this purpose, a new approach is provided based on inserting the second portion of an RSH, i.e., the third and fourth vertical passes and the second connection pass, inside the first portion of the RSH, i.e., inside the first and second vertical passes and the first connection pass. Thereby, the first portion of the RSH is used to protect the second portion of the RSH from radiation from any relevant direction. In other words, the first portion of the RSH, with relatively colder fluid inside, acts as an outer platen, while the second portion of the RSH, with higher temperature fluid inside, acts as the inner platen.
If desired, a complete RSH may comprise several parallel elements, each comprising a series of passes, as described above. By using the above-described RSH, the metal temperatures will be reduced, thus allowing a lower cost tube material, or thinner minimum wall thickness, be used in the RSH. It is also possible, when increased RSH duty is desired, that the superheating system of a boiler comprises two or more radiant superheaters, as described above, connected in series.
The brief description above, as well as further objects, features, and advantages of the present invention will be more fully appreciated by reference to the following detailed description of the currently preferred, but nonetheless illustrative, embodiments of the present invention, taken in conjunction with the accompanying drawings.
The radiant superheater 26 comprises one or more substantially planar, in parallel connected superheating elements 32, each of which comprises a first vertical pass 34, a first connection pass 36, a second vertical pass 38, a third vertical pass 40, a second connection pass 42 and a fourth vertical pass 44. The first connection pass 36 and the second connection pass 42 are advantageously horizontal, but they may, in some applications, also be of another form, for example, half-circles. Steam to be superheated in the radiant superheater 26 is conducted, usually, from a convective superheater (not shown), along a feed pipe 46 to an inlet header 48, which is arranged at the upper end of the first vertical pass 34. Correspondingly, heated steam is conducted via an outlet header 50, arranged at the upper end of the fourth vertical pass 44, along a discharge pipe 52 to the next stage. The next stage may be a finishing superheater 30 or, if the radiant superheater 26 is a finishing superheater, a steam turbine (not shown). Normally, the steam turbine is a high pressure steam turbine, but when the radiant superheater 26 is a reheater, the steam turbine is an intermediate pressure steam turbine.
From the inlet header 48 of the radiant superheater 26, the steam to be superheated is distributed to multiple parallel steam tubes 54 running as U-tubes through the first vertical pass 34, the first connection pass 36 and the second vertical pass 38 to an intermediate header, a so-called first header 56, arranged at the upper end of the second vertical pass 38. From the first header 56, the steam flows via a connecting pipe 58 to a second header 60 arranged at the upper end of the third vertical pass 40. The connecting pipe 58 advantageously comprises a water attemperator 62, by which it is possible to adjust the temperature of the steam to a desired level before it enters the third vertical pass 40. The superheating system may comprise further water attemperators 64 upstream or downstream of the radiant superheater 26. From the second header 60 of the radiant superheater 26, the steam is again distributed to multiple parallel steam tubes 66 running as U-tubes through the third vertical pass 40, the second connection pass 42 and the fourth vertical pass 44, to the outlet header 50.
In accordance with the present invention, the second connection pass 42 is arranged above the first connection pass 36, so that the first connection pass 36 shields the second connection pass 42 from radiation from the lower portion of the furnace 12. In the embodiment shown in
While the invention has been described herein by way of examples in connection with what are at present considered to be the most preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is intended to cover various combinations or modifications of its features, and several other applications included within the scope of the invention as defined in the appended claims. For example, the radiant superheater is advantageously applied in a supercritical or an ultrasupercritical boiler, but it may, if desired, also be applied in a subcritical boiler.
